In vivo wall shear rate in the thoracic aorta of dogs is measured with a flat edged hot film sensor mounted flush with the inner vessel wall. Probe position with respect to the vessel wall is monitored with a pulse echo ultrasonic system. The pulse echo probe is alligned directly opposite the hot film probe to measure relative hot film probe and vessel wall positions. A secondary hot film probe is placed downstream of the primary probe in close proximity to measure blood temperature in the thermal boundary layer established by the primary probe. Reverse flow components are then monitored by the secondary probe. A complete description of blood velocity near the vessel wall, including possible reverse flow is critical to an accurate quantization of wall shear stress. Wall shear stress is computed from shear rate data utilizing a Newtonian model for blood. The relation of in vivo peak and mean wall shear stress values to reported levels enhancing transendothelial lipo-protein flux will aid in the determination of the role of hemodynamic factors in atherogenisis.